A considerable effort has been devoted to recovery of increasingly greater amounts of precious metals from ores, and specifically refractory ores. Gold is found as the native metal widely distributed in minute traces in various minerals, such as certain quartz ores and certain alluvial gravels. The term "ore" as used in this specification includes an ore concentrate and other mineral bearing materials having elements or compounds which would be desirable to recover, but in which the gold is typically not recoverable by basic gravity concentration, or by simple cyanidation.
The method generally employed to extract gold from refractory ores consists of the roasting of sulphide concentrates obtained from crushed ores by flotation or other means of concentration. The roasting is followed by cyanidation.
Gold bearing ores are usually treated by a cyanide extraction process in which the ore is subjected to extraction with sodium cyanide. The cyanide solution then is contacted with a metal such as zinc to cause it to precipitate from solution.
It is generally believed that roasting either liberates the gold from the sulphide minerals or at least exposes the gold to cyanide solutions. In this way, fairly satisfactory results may be obtained from some refractory ores. However, it should be understood that the amount of gold that is recoverable from said refractory ores is very minute. Typically, one ton of a typical refractory ore may contain 80% SiO.sub.2, about 10% of CaCO.sub.3, approximately 5% of FeS.sub.2 (Pyrite) about 1% C but only about 1 ounce of gold.
It is known that the intense roasting of many ores can cause the actual loss of so-called micron gold. The boiling point of gold is sufficiently high that roasting does not actually vaporize the gold; however, very small particles of gold can be lost as an aerosol. Unless an electrostatic precipitation apparatus is installed, and particulates are recovered with great care, as much as 75% of all gold in a particular type of ore can be lost. The economic recovery of gold from low grade ores requires improved processing methods. After sizing the ore, a preliminary oxidizing roast has proven effective in enhancing recovery of gold and reducing recovery costs by limiting cyanide losses. The purpose of the oxidizing roast is to burn the carbon and sulfur contained in the ore, and to oxidize the iron. Additionally, the roast should not destroy the porosity of the ore structure and hinder the access of the leach to the interstitial gold. Unburned carbon left in the processed ore can absorb cyanide in the CIL (Carbon In Leach) recovery processing following the roasting. The unburned carbon in the ore steals cyanide and gold away from the activated carbon added during the leaching step. It has been discovered that an incomplete roast of the ore, at low temperature, can leave gold in the particle core, while a high temperature roast can cause the formation of non-porous hematite. In the latter situation, the gold would become encapsulated, rendering it unobtainable by cyanidation.
The roasting of refractory gold-bearing ores is presently accomplished using rotary kilns and fluid bed reactors. The rotary kiln has disadvantages associated with poor mixing of the process gas with the ore; which limits the oxidation, and temperature control. The fluid bed has the advantages of excellent mixing of ore and oxidizing gas, and good temperature control. The disadvantages of the fluid bed process include high power costs, low gas velocities; and the high cost of injecting pure oxygen. In both methods, the residence time of gold while in the roasting stage is somewhat lengthy, for example, within the fluid bed reactor the time needed is approximately 20 minutes. Obviously, if the time for roasting the gold bearing ores could be appreciably reduced much more material can be processed over a given period of time. In view of the very small amounts of gold extracted from a given amount of material it would be desirable to process the material as quickly as possible.
The invention described here is a novel way of roasting gold ore with advantages over existing processing methods. The invention utilizes a flash furnace system, which offers the unique advantages of very short retention times, high heat transfer rates, excellent oxygen contact with individual particles, and rapid cooling. The primary variables considered in gold ore roasting are temperature, mixing and oxygen contact, and retention time. The flash furnace is a gas suspension processing device, and therefore the solids are mixed thoroughly with the gas stream and carried by the gas stream. As a result of the mixing, oxygen contact and heat transfer are excellent. The short retention time allows burn-off of carbon without the collapse of pore structure sometimes associated with other pyroprocessing methods. This not only enhances carbon removal, but also enhances cyanide contact and removal of the gold ore, and also, of course, increases the amount of ore that can be processed during a given period of time over prior art roasting procedures. In a further embodiment of the invention, the retention time can be increased if necessary by recirculating a portion of the collected product back to the flash furnace. This requires little additional heat to accomplish, since the hot material is recirculated directly from the hot collection cyclone.